When you say cooking, do you mean any kind of food prep, or only the kind that involves altering the food by heating it? I think spinach is my favorite to eat of the first group, but if we're excluding raw uses it might be kale.

I never eat any of the second group!

This year I tried growing a new plant called kalettes. I think it is a cross between kale and brussels sprouts. All summer it grew a bigger and bigger stalk, with enormous leathery kale leaves. It promised tiny tender heads of kale in the leaf axils, where brussels sprouts would be, but I had almost given up hope when they finally showed up in November. I'm still harvesting a few now, after several snows and many bitter cold nights. They are both tender and nutty. Definitely worth garden space!